Airbags for vehicles are intended to protect an occupant's body such as a face, head and the like in the event of a crash by actuating a sensor following an impact, generating a high-temperature and high-pressure gas, and instantaneously inflating an airbag with this gas. In recent years, airbags for vehicles have been widely used as one of safety devices and have been developed in their practical use not only for a driver seat and a passenger seat but also as knee airbags, side airbags, curtain airbags, etc., and installment of a plurality of airbags is now becoming common.
In side airbags, curtain airbags, etc. for which an internal pressure retention performance is particularly required, a coated woven fabric coated with a synthetic rubber such as silicone has been used because its heat resistance, air insulating property (low air permeability) and flame-retardancy are high. In recent years, the amount of coating is desired to be reduced from the viewpoint of weight reduction and improvement of package ability of airbags.
In general, airbags are cut by cutting methods such as knife cutting, melt cutting with a laser, or the like. Since coated woven fabrics have a smaller number of frayed thread compared with those of uncoated fabrics, the knife cutting is used for coated woven fabrics. However, if the coating amount of the coated fabric decreases, frayed threads increase when the coated fabric is cut with a knife or sewn, and it is necessary to increase the size of a cut part to make a margin rate larger. Therefore, coated fabrics hardly causing thread fraying even with a small amount of coating are demanded.
Although resin such as a synthetic rubber that is a coated material of the coated woven fabrics has been hitherto used as a coating agent after making into a solution using an organic solvent, there is a problem that the organic solvent is vaporized so that the environment of particularly the working place is polluted, and therefore a non-solvent-based silicone has been used mainly at present. However, there is a limit to the reduction in a coating amount because a solid content ratio in a coating agent made of a non-solvent-based silicone is 100%, and it has been technically difficult to reduce the coating amount to 10 g/m2 or less.
Patent Document 1 discloses a technology in which when a solid powder dispersed in water under the presence of a surfactant is used in a silicone aqueous emulsion, the added amount of the solid powder is made within a range from 0.1 parts by mass to less than 5 parts by mass. Although the coating amount of 10 g/m2 or less is achieved and a rate of 60 m/min or lower is also achieved in a flammability test (JIS D 1201; Horizontal Method), it is not sufficient as a rate of flammability required for airbags for vehicles. Furthermore, there is no description therein for air permeability and thread fraying when the fabric is cut with a knife or sewn.
Patent Documents 2 and 3 disclose a technology in which a reduced coating amount is realized by a method of impregnating with a water-dispersible resin. Although the coating amount of 10 g/m2 or less is achieved, and both of flammability and initial air permeability satisfy the performance required for airbags for vehicles, there is no description for thread fraying when the fabric is cut with a knife or sewn. Therefore, it is a fact that the extensive study has not been made for an optimum base fabric satisfying initial air permeability and flammability even with a small coating amount of 10 g/m2 or less and also from the viewpoint of thread fraying when the fabric is cut with a knife or sewn.